Blind fasteners are used in a variety of applications wherein access to the blind-side of panels being connected together is extremely limited or in some cases not possible. Various particular applications impose unique, stringent requirements for which the fastener must be particularly adapted. One of these applications is in the aerospace industry wherein the fasteners are required to securely fasten the panels, yet resist losing their clamping force during the service life of the aircraft. Hence, the fastener must be able to withstand the severe stresses and vibrations imposed upon them by the often harsh environment in which they are used without loosening.
One type of blind fastener is a three piece assembly having a threaded core bolt, a body and a sleeve. The body has a conical inner end and a threaded bore for receiving the core bolt. The core bolt has an enlarged head and a threaded stem for threadably engaging the body. The sleeve is unthreaded and located between the bolt head and the conical end of the body. The three piece assembly is inserted, bolt head first, into aligned openings of panels to be fastened together such that the bolt head, the sleeve and the conical end of the body are located inwardly of the blind side of the interior panel. The fastener may be set by rotating the core bolt, which causes axial movement of the bolt head and sleeve toward the panels. The outer end of the sleeve expands over the conical end of the body until it becomes seated firmly against the blind side of the interior panel. A further detailed description of this type of blind fastener is set forth in U.S. Pat. Nos. 4,457,652, 5,066,179 and 5,238,342, which are incorporated by reference herein.
While the above-described type of blind fastener functions quite well, it does have a disadvantage. In particular, the core bolt and body are threaded in the shear zone, i.e., the plane denned by the overlapping panels being fastened together, resulting in lower fatigue capabilities in both tension and shear loading of the fastener.
Another type of blind fastener overcomes the above problem. It is a three piece assembly having a core bolt, a body and a threaded sleeve. In this case, the body has an unthreaded bore for receiving the core bolt and the core bolt is threaded at its inner end only for receiving the threaded sleeve. Thus, the fastener is unthreaded in the shear zone improving its fatigue capabilities. The three piece assembly is inserted, sleeve first, through the aligned openings such that the sleeve, the threaded inner end of the core bolt, and the conical end of the body are located inwardly of the blind side of the interior panel. The fastener may then be set by rotating the core bolt, which causes axial movement of the sleeve toward the panels, bringing the sleeve into engagement with the blind side of the interior panel. A further detailed description of this type of blind fastener is set forth in U.S. Pat. Nos. 2,971,425, 3,063,329, 3,345,900 and 4,579,491.
This second type of blind fastener has its own disadvantages. In particular, to work properly, the sleeve must be relatively soft at its end adjacent the blind side of the interior panel to permit its radial expansion over the conical end of the body, yet its other end must be hard to provide the necessary strength to prevent stripping during setting of the fastener. Requiring the sleeve to have both a soft portion and a hard portion, however, restricts the types of treatment that the sleeve may undergo, which would otherwise permit the sleeve as a whole to be made softer or harder. As a result, the tensile strength and yield strength of the fastener are not as great as they could be and its fatigue capabilities are lower.
It should therefore be appreciated that there is still a need for a blind fastener having a deformable sleeve that has increased strength, but which is still relatively easy to make and simple to install.